Electronic devices with yielding substrates

ABSTRACT

In accordance with certain embodiments, a semiconductor die is adhered directly to a yielding substrate with a pressure-activated adhesive notwithstanding any nonplanarity of the surface of the semiconductor die or non-coplanarity of the semiconductor die contacts.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 61/359,467, filed Jun. 29, 2010, U.S. ProvisionalPatent Application No. 61/363,179, filed Jul. 9, 2010, U.S. ProvisionalPatent Application No. 61/376,707, filed Aug. 25, 2010, U.S. ProvisionalPatent Application No. 61/390,128, filed Oct. 5, 2010, U.S. ProvisionalPatent Application No. 61/393,027, filed Oct. 14, 2010, U.S. ProvisionalPatent Application No. 61/433,249, filed Jan. 16, 2011, U.S. ProvisionalPatent Application No. 61/445,416, filed Feb. 22, 2011, and U.S.Provisional Patent Application No. 61/447,680, filed Feb. 28, 2011. Theentire disclosure of each of these applications is hereby incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention generally relates to electronic devices, and morespecifically to array-based electronic devices.

BACKGROUND

Discrete light sources such as light-emitting diodes (LEDs) are anattractive alternative to incandescent light bulbs in illuminationdevices due to their higher efficiency, smaller form factor, longerlifetime, and enhanced mechanical robustness. However, the high cost ofLEDs and associated heat-sinking and thermal-management systems havelimited the widespread utilization of LEDs, particularly in generallighting applications.

The high cost of LED-based lighting systems has several contributors.LEDs are typically encased in a package and multiple packaged LEDs areused in each lighting system to achieve the required light intensity. Inorder to reduce costs, LED manufacturers have developed high-power LEDs,which can emit relatively higher light intensities by operating atrelatively higher currents. While reducing the package count, these LEDsrequire relatively higher-cost packages to accommodate the highercurrent levels and to manage the significantly higher heat levels thatresult. The higher heat loads and currents, in turn, require moreexpensive thermal-management and heat-sinking systems—for example,thermal slugs in the package, ceramic or metal submounts, large metal orceramic heat sinks, metal core printed circuit boards and the like—whichalso add to the cost as well as to size of the system. Higher operatingtemperatures also lead to shorter lifetimes as well as reducedreliability. Finally, LED efficacy typically decreases with increasingdrive current, so operation of LEDs at relatively higher currentsresults in a relative reduction in efficacy as compared withlower-current operation. In order to support high-current operation, theLED chip (inside the package) requires relatively larger contact areas.In addition, high-power LEDs often have a current-blocking layer underthe contacts to prevent light emission in those areas. The largercontact areas and current-blocking layer diminish the light-emittingarea of the chip, resulting in reduced efficiency, fewer chips per waferand increased cost.

Contact size is further limited by the method used to connect the LEDchip to the package, another substrate or other supporting components.Most commonly, LED chips are interconnected using wire bonding. Thewire-bonding process requires a certain minimum contact area,independent of current level. As a result, even in low-current LEDs, thecontact size cannot be reduced below the minimum required for wirebonding. Another common approach for connection of the LED chip to thepackage is to use agents such as solder or conductive adhesives to bonda LED to a package, submount or substrate. These agents may also berelatively expensive and require complicated processes to control theirdispersion so as to prevent the contacts of the LED from shortingtogether and rendering the device inoperative; this is particularly soas device geometries (for example, spacing between contacts) anddimensions continue to shrink.

One recent advance facilitating the connectivity of LEDs to a variety ofsubstrates is anisotropically conductive adhesive (ACA), which enableselectrical interconnection in one direction (e.g., vertically between adevice contact and a substrate contact), but prevents it in otherdirections (e.g., horizontally between contacts on a device or betweencontracts on a substrate). State-of-the-art ACAs are pressure-activated,and thus require provision of “stud bumps” or other metallic projectionson the surface to which the LED is to be bonded or on the LED bond padsin order to create the anisotropic electrical conductivity and promoteadhesion. While other, non-pressure-activated types of ACA exist (e.g.,ZTACH available from SunRay Scientific of Mt. Laurel, N.J., for which amagnetic field rather than pressure is applied during curing in order toalign magnetic and conductive “columns” in the desired conductiondirection), such ACAs are less common and require additional, andpotentially expensive, equipment (e.g., magnets).

As known in the art, a pressure-activated ACA typically comprises anadhesive base, e.g., an adhesive or epoxy material, containing“particles” (e.g., spheres) of a conductive material or of an insulatingmaterial coated with a conductive material (such as metal or aconductive material coated with an insulating material. FIG. 1 depicts aconventional use of pressure-activated ACA to connect an electronicdevice to a substrate. As shown, the electronic device 100 havingmultiple contacts 110 has been adhered and electrically connected to asubstrate 120 via use of an ACA 130. The ACA 130 comprises an adhesivebase 140 containing a dispersion of particles 150 that are at leastpartially conductive. As mentioned above and as shown in FIG. 1,conventionally, the use of ACA requires the target substrate to containstud bumps (which typically have a thickness of at least 30 μm-50 μm),or other conductive structures projecting from the substrate, oppositethe device contacts to be bonded in order to achieve adequate bondingand electrical connectivity between the device and the electricalinterconnects on the substrate. That is, in the context of FIG. 1, theadhesion and electrical connection of contacts 110 to electrical traces160 (the thickness of which has been exaggerated for clarity) onsubstrate 120 requires the presence of stud bumps 170. As shown, theconductive particles 150 provide electrical connectivity between eachcontact 110 and its respective trace 160, but are dispersed within base140 at a sufficiently low density such that an electrical connection isnot formed between the contacts 110 and/or the traces 160. The studbumps 170 provide not only a portion of the electrical connection, butalso a solid platform against which the particles 150 are compressed,sharply increasing the conductivity of the ACA 130 and enabling theelectrical connection therethrough (but not across the uncompressed ACAbetween the contact/stud bump pairs). In an alternate geometry, the studbumps may be attached to contacts 110. It should be noted that othertechniques involving ACAs are possible, and the present invention is notlimited by the particular mode of operation of the ACA.

However, the use of stud bumps or equivalent conductive structures maybe problematic and costly in many applications. Particularly as deviceand device-contact dimensions continue to decrease, stud bumps arefrequently too large for connection to individual contacts. Formation ofstud bumps also necessarily entails the formation of topography on thesubstrate, a complicated and expensive process, particularly when devicecontacts are non-coplanar (as stud bumps of a variety of heights arerequired). Furthermore, in applications utilizing unpackagedsemiconductor die (e.g., bare-die LEDs), bonding of the device to studbumps may result in deleterious localized stress (e.g., if the die bowsbetween stud bumps due to the applied bonding pressure). Finally, use ofstud bumps or similar structures may result in thermal-expansionmismatch (and concomitant stress) between the bumps and the substrate orbonded die.

However, without stud bumps or other projecting structures, bonding asemiconductor die to conventional substrates will not result in areliable electrical connection therebetween, particularly if thecontacts on the semiconductor die are non-coplanar. FIG. 2 depicts acommon device environment that illustrates the problem. As shown, a LEDdie 200 features a contact 210 to an n-doped layer 220 and a contact 230to a p-doped layer 240. A portion of the p-doped layer 240 has beenremoved to enable formation of contact 210 over the n-doped layer 220,rendering contacts 210 and 230 non-coplanar. In FIG. 2, an attempt hasbeen made to bond LED die 200 to a conventional substrate 120 (e.g., aprinted circuit board), which is substantially rigid and non-deformable.Due at least in part to the non-coplanarity between contact 210 andcontact 230, the particles 150 of the pressure-activated ACA 130establish electrical contact in the compression zone between contact 230and its corresponding trace 160-1, but, in the absence of stud bumps, asimilar electrical connection cannot be formed between contact 210 andits corresponding trace 160-2 due to the absence of sufficientcompression. Even if a temporary electrical connection is initiallyformed between contact 210 and trace 160-2, upon cure of the ACA 130and/or during operation, the ACA 130 may expand or contract, resultingin the loss of electrical contact and inoperability of LED die 200. Suchexpansion and/or contraction may also occur during operation, forexample from ambient heating or self-heating from operation, resultingin unreliable operation.

In view of the foregoing, a need exists for systems and proceduresenabling the low cost reliable bonding of various semiconductor dies(e.g., LED dies and solar cell dies) directly to a substrate'selectrical traces via pressure-activated adhesives without the use ofstud bumps or similar structures and low cost, reliable LED-basedlighting systems based on such systems and processes.

SUMMARY

In accordance with certain embodiments, one or more semiconductor diesare attached to a flexible and/or deformable substrate with apressure-sensitive adhesive (e.g., an ACA) without the use ofintervening stud bumps or similar structures. The substrate is able tolocally yield to compression force and form a mechanically strong andelectrically conductive connection to the semiconductor-die contacts,notwithstanding any non-coplanarity of the contacts. In someembodiments, the substrate is “flexible” in the sense of being pliant inresponse to a force and resilient, i.e., tending to elastically resumean original configuration upon removal of the force. A substrate may be“deformable” in the sense of conformally yielding to a force, but thedeformation may or may not be permanent; that is, the substrate may notbe resilient. Flexible materials used herein may or may not bedeformable (i.e., they may elastically respond by, for example, bendingwithout undergoing structural distortion), and deformable substrates mayor may not be flexible (i.e., they may undergo permanent structuraldistortion in response to a force). The term “yielding” is herein usedto connote a material that is flexible or deformable or both.

The use of the yielding substrate simplifies the bonding andsubstrate-preparation procedures, and also facilitates deployment of thesemiconductor dies in environments and/or applications unsuitable forrigid substrates. The substrate may even be substantially transparent,further broadening the scope of potential applications for whichembodiments of the invention may be utilized. Since the yieldingsubstrate enables the inexpensive and simple fabrication of assembliesfeaturing arrays of semiconductor dies, embodiments of the invention mayeven be advantageously utilized in applications where substrate rigiditymay be preferred. For example, the flexible substrate(s) andsemiconductor dies may be attached to and/or mounted withinsubstantially rigid frames or other apparatus that provide structuralsupport. In one such embodiment, one or more arrays of light-emittingsemiconductor dies on one or more yielding substrates may be mountedwithin a rigid frame to form a lighting assembly for applications suchas backlighting and general illumination.

An advantage of the present invention is the ability to replace today'sfluorescent fixtures (e.g., standard linear fluorescent troffers), whichcan be inefficient due to optical losses, with designs that minimizeoptical loss. Moreover, fluorescent lamps contain mercury, which can beenvironmentally deleterious unless disposed of properly (andexpensively). Embodiments of the present invention have luminousefficacies greater than those associated with conventional fluorescentfixtures. More generally, LED lighting has the potential to dramaticallyreduce energy consumption due to its much higher efficiency relative toincandescent, halogen and compact fluorescent lamps.

In an aspect, embodiments of the invention feature an electronic devicecomprising a semiconductor die having first and second distinctnon-coplanar contacts on a first surface thereof, and a yieldingsubstrate having first and second conductive traces on a first surfacethereof. The first and second conductive traces are separated on thesubstrate by a gap therebetween. The first and second contacts areadhered to and in electrical contact with, respectively, the first andsecond conductive traces with a pressure-activated adhesive materialnotwithstanding the non-coplanarity of the first and second contacts,and without electrically bridging the traces or the contacts. In someembodiments, the substrate is flexible but not deformable; in otherembodiments, the substrate is deformable but not flexible; while instill other embodiments, the substrate is both flexible and deformable.

The semiconductor die may comprise a LED die, e.g., an inorganic LEDdie. Alternatively, the semiconductor die may comprise a laser and maycomprise a semiconductor material comprising or consisting essentiallyof at least one of GaN, AlN, InN, or an alloy or mixture thereof; or asemiconductor material comprising or consisting essentially of at leastone of silicon, GaAs, InAs, AlAs, InP, GaP, AlP, InSb, GaSb, AlSb, ZnO,or an alloy or mixture thereof.

In various embodiments, the adhesive material comprises or consistsessentially of an ACA electrically connecting the first contact only tothe first trace and the second contact only to the second trace. Aportion of the ACA may disposed in the gap to substantially isolate thefirst contact from the second contact. In some embodiments, the adhesivematerial comprises a substantially isotropic adhesive electricallyconnecting the first contact only to the first trace and the secondcontact only to the second trace, and the device further comprises anon-conductive adhesive material disposed in the gap. The first andsecond traces may have substantially uniform and substantially equalthicknesses.

In some embodiments, the device further comprises a reflective materialover at least a portion of the first surface of the semiconductor die.An offset between the first and second contacts along a dimensionsubstantially perpendicular to the first surface of the semiconductordie may be at least 0.25 μm. In various embodiments, the semiconductordie is unpackaged. The yielding substrate may comprise a localizeddeformation between the first and second traces, whereby the distancebetween the first contact and the substrate is substantially equal tothe distance between the second contact and the substrate.

In general, the semiconductor die will extend across the gap between thefirst and second traces, and in some embodiments, a second semiconductordie, proximate the semiconductor die, also extends across the gapbetween the first and second traces. In some embodiments, the first andsecond conductive traces comprise a conductive ink; and the conductiveink may comprise, for example, silver, gold, aluminum, chromium, copper,and/or carbon. In various embodiments, the reflectivity of the substratefor a wavelength emitted by the semiconductor die is greater than 80%,whereas in other embodiments, a transmittance of the substrate for awavelength emitted by the semiconductor die is greater than 80%. Thesubstrate may comprise or consist essentially of polyethylenenaphthalate, polyethylene terephthalate, polycarbonate,polyethersulfone, polyester, polyimide, polyethylene, and/or paper. Thegap between the first and second traces may range between approximately25 μm and approximately 1000 μm. An advantage of the invention is thatthere need be no heat sink in thermal communication with thesemiconductor die.

In various embodiments, disposed over and at least partially surroundingthe semiconductor die is a phosphor material for converting at least aportion of light emitted by the semiconductor die to light of adifferent wavelength. There may be a second substrate disposed over theyielding substrate and the first and second conductive traces, with thesecond substrate comprising an opening defined thereby; in such cases,the semiconductor die and the phosphor material may be disposed in theopening. Moreover, a transparent film may be disposed over the openingin the second substrate, which may be yielding.

In some embodiments, an optically transparent material is disposedbetween the semiconductor die and the phosphor material. A reflectivesurface for reflecting converted light toward the yielding substrate maybe disposed over the phosphor material.

In another aspect, embodiments of the invention relate to an electronicdevice comprising a semiconductor die having first and secondspaced-apart contacts on a first surface thereof, and a yieldingsubstrate having first and second conductive traces on a first surfacethereof in a bonding region; the first and second conductive tracesdefining a gap therebetween. Furthermore, the first and second contactsare adhered to and in electrical contact with, respectively, the firstand second conductive traces with a pressure-activated adhesive materialwithout electrically bridging the traces or the contacts; and at leastin the bonding region, the height of the first and second traces abovethe first surface of the substrate does not exceed 10 μm (or, in someembodiments, does not exceed 5 μm, or in other embodiments, does notexceed 1 μm).

In still another aspect, the invention pertains to a method of formingan electronic device. In various embodiments, the method comprisesproviding a yielding substrate having first and second conductive traceson a first surface thereof in a bonding region, where the first andsecond conductive traces are separated on the substrate by a gaptherebetween. With a pressure-activated adhesive material, the first andsecond contacts of a semiconductor die are adhered to the first andsecond traces, respectively, by applying pressure to at least one of theyielding substrate or the semiconductor die, thereby establishingelectrical connection between (i) the first contact and the first traceand/or (ii) the second contact and the second trace, but withoutelectrically bridging the traces or the contacts.

In some embodiments, the substrate is flexible but not deformable; inother embodiments, the substrate is deformable but not flexible; whilein still other embodiments, the substrate is both flexible anddeformable. Providing the substrate may, for example, comprise printingthe first and second traces thereon. The adhesive may, in someembodiments, be cured. The first and second contacts may be co-planar ornon-coplanar. Applying pressure to the yielding substrate and/or thesemiconductor die may comprise compressing the substrate and thesemiconductor die between a substantially rigid surface and asubstantially compliant surface to adhere the first and second contactsto the first and second traces notwithstanding the non-coplanaritybetween the first and second contacts. Prior to adhering, the adhesivematerial may be provided on the first and second contact and/or thefirst and second traces. Providing the adhesive material may comprisedispensing the adhesive material in substantially liquid form. Invarious embodiments, the adhesive material comprises or consistsessentially of an ACA. A non-conductive adhesive material may be formedover the yielding substrate within the gap.

In some embodiments, the method further comprises forming a phosphormaterial over at least a portion of the semiconductor die; the phosphormaterial converts at least a portion of light emitted by thesemiconductor die to light of a different wavelength. A second substratemay, if desired, be disposed on the first surface of the yieldingsubstrate; the second substrate defines an opening therethrough in whichthe semiconductor die is disposed. The opening may be at least partiallyfilled with a phosphor material such that the phosphor material at leastpartially surrounds the semiconductor die.

A second substrate, comprising a depression in which the semiconductordie is disposed, may be formed on the first surface of the yieldingsubstrate. A phosphor material may be disposed over a surface of thedepression, and/or may be disposed between the semiconductor die and areflective surface for reflecting the converted light toward theyielding substrate. The semiconductor die may be unpackaged, and may be,for example, a LED, e.g., an inorganic LED die. Alternatively, thesemiconductor die may comprise or be a laser.

Providing the yielding substrate and adhering the contacts to the tracesmay be performed in a roll-to-roll process, for example. In variousembodiments, using an adhesive material, the first and second contactsof a second semiconductor die are adhered to third and fourth conductivetraces disposed on a second surface of the yielding substrate opposingthe first surface. In some embodiments, the first and second contactsare substantially coplanar and, at least in the bonding region, theheight of the first and second traces above the first surface of thesubstrate does not exceed 10 μm.

In yet another aspect, the invention pertains to an electronic devicecomprising, in various embodiments, a semiconductor die comprising aplurality of active semiconductor layers and a plurality of contacts. Afirst and a second of the active semiconductor layers collectivelydefine a non-planar first surface to which a first and a second of thecontacts are joined. The device further comprises a yielding substratehaving first and second conductive traces on a first surface thereof,the first and second conductive traces being separated on the substrateby a gap therebetween. The first and second contacts are adhered to andin electrical contact with, respectively, the first and secondconductive traces with a pressure-activated adhesive materialnotwithstanding the non-planarity of the first surface of thesemiconductor die, and without electrically bridging the traces or thecontacts. The semiconductor die may comprise or consist of asemiconductor substrate on which the plurality of active semiconductorlayers is disposed. The plurality of active semiconductor layers maycomprise or consist of a light-emitting quantum well disposed betweenthe first and second active semiconductor layers.

These and other objects, along with advantages and features of theinvention, will become more apparent through reference to the followingdescription, the accompanying drawings, and the claims. Furthermore, itis to be understood that the features of the various embodimentsdescribed herein are not mutually exclusive and can exist in variouscombinations and permutations. As used herein, the term “substantially”means±10%, and in some embodiments, ±5%.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a schematic illustration of a semiconductor die bonded to studbumps on a substrate via a pressure-activated adhesive in accordancewith the prior art;

FIG. 2 is a schematic illustration of a semiconductor die bonded to asubstrate via a pressure-activated adhesive in the absence of studbumps, depicting the resulting unreliable or absent electricalconnection;

FIGS. 3A and 3B are schematic illustrations of a semiconductor die indifferent stages of processing, in accordance with various embodimentsof the invention;

FIG. 3C is a schematic illustration of a semiconductor die, inaccordance with an embodiment of the invention;

FIG. 4 is a schematic illustration of a yielding substrate utilized inaccordance with various embodiments of the invention;

FIGS. 5A and 5B are schematic illustrations of a semiconductor dieadhered to a yielding substrate in accordance with various embodimentsof the invention;

FIG. 6A is a schematic top view of an electronic device featuringmultiple semiconductor dies adhered to a yielding substrate as shown inFIG. 5, in accordance with various embodiments of the invention;

FIG. 6B is a magnified top view of multiple semiconductor dies adheredbetween conductive traces in an electronic device similar to thatdepicted in FIG. 6A, in accordance with various embodiments of theinvention;

FIGS. 7A and 7B are schematic top views of layouts of electrical tracesutilizes in electronic devices in accordance with various embodiments ofthe invention;

FIGS. 8A-8D are schematic cross-sections of the integration of phosphorwith semiconductor dies adhered to a substrate in accordance withvarious embodiments of the invention;

FIGS. 9A-9D are schematic cross-sections of the integration of phosphorwith semiconductor dies adhered to a substrate in accordance withvarious other embodiments of the invention;

FIG. 10 is a schematic cross-section of an electronic device featuring asemiconductor die and a remote phosphor in accordance with variousembodiments of the invention;

FIG. 11 is a schematic cross-section of an electronic device featuring asemiconductor die and a second substrate incorporating a remote phosphorin accordance with various embodiments of the invention;

FIG. 12A is an isometric view of an electronic module incorporatinglight-emitting semiconductor dies in accordance with various embodimentsof the invention;

FIG. 12B is a magnified view of a portion of the module depicted in FIG.12A;

FIG. 13 is an exploded view of portions of the electronic module of FIG.12A;

FIG. 14 is an isometric view of an electronic module incorporatingmultiple substrates each with semiconductor dies adhered thereto, inaccordance with various embodiments of the invention;

FIG. 15 is an exploded view of portions of the electronic module of FIG.14;

FIG. 16A is an isometric view of an electronic module incorporatinglight-emitting semiconductor dies and a sensor in accordance withvarious embodiments of the invention;

FIG. 16B is a schematic view of a network of electronic modules likethat depicted in FIG. 16A;

FIGS. 17, 18A and 18B are partially exploded cross-sections ofelectronic modules for backlighting applications in accordance withvarious embodiments of the invention;

FIGS. 19 and 20 are partially exploded cross-sections of electronicmodules for general illumination in accordance with various embodimentsof the invention;

FIG. 21 is a bottom view of an electronic module incorporating multiplesubstrates with semiconductor dies adhered thereto, in accordance withvarious embodiments of the invention;

FIGS. 22A and 22B are, respectively, a bottom view and a schematiccross-section of portions of the module of FIG. 21;

FIG. 23 is a schematic cross-section of the module of FIG. 21 insertedinto a mechanical support frame;

FIGS. 24A and 24B are, respectively, a top isometric view and a bottomisometric view of an electronic module utilized as a retrofit for aluminaire in accordance with various embodiments of the invention;

FIG. 25 is a magnified cross-section of portions of the module depictedin FIGS. 24A and 24B;

FIGS. 26 and 27 are, respectively, a partially exploded isometric topview and an unexploded isometric top view of an electronic moduleutilized as a retrofit for a luminaire in accordance with variousembodiments of the invention;

FIG. 28A is a bottom isometric view of the module depicted in FIGS. 26and 27 incorporating an optional diffuser sheet; and

FIG. 28B is a bottom isometric view of the module depicted in FIGS. 26and 27 without the optional diffuser sheet.

DETAILED DESCRIPTION

Refer first to FIGS. 3A and 3B, which depict an exemplary semiconductordie 300 for use in various embodiments of the present invention.Semiconductor die 300 typically includes a substrate 310 with one ormore semiconductor layers 320 disposed thereover. In this exemplaryembodiments, semiconductor die 300 represents a light-emitting devicesuch as a LED or a laser, but other embodiments of the invention featureone or more semiconductor die with different or additionalfunctionality, e.g., processors, sensors, detectors, and the like.Non-LED die may or may not be bonded as described herein, and may havecontact geometries differing from those of the LEDs; moreoever, they mayor may not have semiconductor layers disposed over a yielding substrateas discussed below.

Substrate 310 may include or consist essentially of one or moresemiconductor materials, e.g., silicon, GaAs, InP, GaN, and may be dopedor substantially undoped (e.g., not intentionally doped). In someembodiments substrate 310 includes or consists essentially of sapphireor silicon carbide. Substrate 310 may be substantially transparent to awavelength of light emitted by the semiconductor die 300. As shown for alight-emitting device, semiconductor layers 320 may include first andsecond doped layers 330, 340, which preferably are doped with oppositepolarities (i.e., one n-type doped and the other p-type doped). One ormore light-emitting layers 350, e.g., one or more quantum wells, may bedisposed between layers 330, 340. Each of layers 330, 340, 350 mayinclude or consist essentially of one or more semiconductor materials,e.g., silicon, InAs, AlAs, GaAs, InP, AlP, GaP, InSb, GaSb, AlSb, GaN,AlN, InN, and/or mixtures and alloys (e.g., ternary or quaternary, etc.alloys) thereof. In preferred embodiments, semiconductor die 300 is aninorganic, rather than a polymeric or organic, device. As referred toherein, semiconductor dies may be packaged or unpackaged unlessspecifically indicated (e.g., a bare-die LED is an unpackagedsemiconductor die). In some embodiments, substantially all or a portionof substrate 310 is removed prior to or after the bonding ofsemiconductor die 300 described below. Such removal may be performed by,e.g., chemical etching, laser lift-off, mechanical grinding and/orchemical-mechanical polishing or the like. In some embodiments all or aportion of substrate 310 may be removed and a second substrate—e.g., onethat is transparent to or reflective of a wavelength of light emitted bysemiconductor die 300—is attached to substrate 310 or semiconductorlayers 320 prior to or after the bonding of semiconductor die 300 asdescribed below. In some embodiments substrate 310 comprises silicon andall or a portion of silicon substrate 310 may be removed prior to orafter the bonding of semiconductor die 300 described below. Such removalmay be performed by, e.g., chemical etching, laser lift off, mechanicalgrinding and/or chemical-mechanical polishing or the like.

As shown in FIG. 3B, in preferred embodiments semiconductor die 300 ispatterned and etched (e.g., via conventional photolithography and etchprocesses) such that a portion of layer 330 is exposed in order tofacilitate electrical contact to layer 330 and layer 340 on the sameside of semiconductor die 300 (and without, for example, the need tomake contact to layer 330 through substrate 310 or to make contact tolayer 330 with a shunt electrically connecting a contact pad over layer340 to layer 330). One or more portions of layers 340, 350 are removed(or never formed) in order to expose a portion of layer 330, and thusFIG. 3B depicts a surface 360 of semiconductor die 300 that isnon-planar, i.e., contains exposed portions non-coplanar with eachother. Surface 360 corresponds to the outer surface of semiconductor die300, including any contour or topography resulting from portions oflayers not being present. In order to facilitate electrical contact tosemiconductor die 300, discrete electrical contacts 370, 380 are formedon layers 330, 340, respectively. Electrical contacts 370, 380 may eachinclude or consist essentially of a suitable conductive material, e.g.,one or more metals or metal alloys conductive oxides, or other suitableconductive materials and are generally non-coplanar (particularly inembodiments when having approximately equal thicknesses), as depicted inFIG. 3B. In some embodiments, the vertical offset between exposedsurfaces of layer 330 and layer 340 (and/or between contacts 370, 380)is at least 0.25 micrometers (μm), at least 1 μm, at least 3 μm, or evengreater.

In some embodiments, semiconductor die 300 has a square shape, while inother embodiments semiconductor die 300 has a rectangular shape. In somepreferred embodiments, to facilitate bonding (as described below)semiconductor die 300 may have a shape with a dimension in one directionthat exceeds a dimension in an orthogonal direction (e.g., a rectangularshape), and have an aspect ratio of the orthogonal directions (length towidth, in the case of a rectangular shape) of semiconductor die 300greater than about 1.2:1. In some embodiments, semiconductor die 300 hasan aspect ratio greater than about 2:1 or greater than 3:1. The shapeand aspect ratio are not critical to the present invention, however, andsemiconductor die 300 may have any desired shape.

In some embodiments, semiconductor die 300 has one lateral dimensionless than 500 μm. Exemplary sizes of semiconductor die 300 may include˜250 μm×˜600 μm, ˜250 μm×˜400 μm, ˜250 μm×˜300 μm, or ˜225 μm×˜175 μm.In some embodiments, semiconductor die 300 comprises a small LED die,also referred to as a “MicroLED.” A MicroLED generally has one lateraldimension less than about 300 μm. In some embodiments semiconductor die300 has one lateral dimension less than about 200 μm or even less thanabout 100 μm. For example, a MicroLED may have a size of ˜225 μm×˜175 μmor ˜150 μm×˜100 μm or ˜150 μm×˜50 μm. In some embodiments, the surfacearea of the top surface of a MicroLED is less than 50,000 μm² or lessthan 10,000 μm².

Because preferred embodiments facilitate electrical contact to contacts370, 380 via use of a conductive adhesive rather than, e.g., wire bonds,contacts 370, 380 may have a relatively small geometric extent sinceadhesives may be utilized to contact even very small areas impossible toconnect with wires or ball bonds (which typically require bond areas ofat least 80 μm on a side). In various embodiments, the extent of one orboth of contacts 370, 380 in one dimension (e.g., a diameter or sidelength) is less than approximately 100 μm, less than approximately 70μm, less than approximately 35 μm, or even less than approximately 20μm.

Particularly if semiconductor die 300 includes or consists essentiallyof a light-emitting device such as a LED or laser, contacts 370, 380 maybe reflective (at least to some or all of the wavelengths emitted bysemiconductor die 300) and hence reflect emitted light back towardsubstrate 310. In some embodiments, a reflective contact 380 covers aportion or substantially all of layer 340, while a reflective contact370 covers a portion or substantially all of layer 330. In addition toreflective contacts, a reflector 390 (not shown in subsequent figuresfor clarity) may be disposed between or above portions of contacts 370,380 and over portions or substantially all of layer 340 and 330.Reflector 390 is reflective to at least some or all wavelengths of lightemitted by semiconductor die 300 and may comprise various materials. Inone embodiment reflector 390 is non-conductive so as not to electricallyconnect contacts 370, 380. Reflector 390 may be a Bragg reflector.Reflector 390 may comprise one or more conductive materials, e.g.,metals such as silver, gold, platinum, etc. Instead of or in addition toreflector 390, exposed surfaces of semiconductor die except for contacts370, 380 may be coated with one or more layers of an insulatingmaterial, e.g., a nitride such as silicon nitride or an oxide such assilicon dioxide. In some embodiments contacts 370, 380 comprise a bondportion for connection to traces 410 and a current-spreading portion forproviding more uniform current through semiconductor die 300, and insome embodiments, one or more layers of an insulating material areformed over all or portions of semiconductor die 300 except for the bondportions of contacts 370, 380. FIG. 3C shows a schematic of die 300 withinsulating material 395 covering the surface of semiconductor die 300except for contacts 370, 380. Insulating material 395 may comprise orconsist essentially of, for example, silicon nitride, silicon oxideand/or silicon dioxide. Such insulating material 395 may cover all orportions of the top and sides of semiconductor die 300 as well asportions of the top and sides of layers 330, 340 and 350. Insulatingmaterial 395 may act to prevent shorting between contacts 370 and 380 orbetween traces 410 (see FIG. 4), or both during and after the bondingoperation with adhesive.

With reference to FIGS. 3A, 3B, 3C and 4, semiconductor die 300 operatesat a current and temperature sufficiently low to prevent melting orother damage to adhesive 510 or to the substrate 400. For example, theoperating current of semiconductor die 300 may be less thanapproximately 50 mA, 10 mA or in some embodiments less than 5 mA. Insome embodiments the operation current is between approximately 1 mA andapproximately 5 mA. The junction temperature of semiconductor die 300during operation may not exceed approximately 100° C., 90° C. or may notexceed 80° C. It should be understood, however, that this is notcritical to the present invention and in other embodiments the junctiontemperature may be any value that does not damage or otherwise adverselyaffect substrate 400, adhesive 510 or other components of the system.Substrates such as PEN, for example, can withstand higher temperaturesthan PET, and those of skill in the art can straightforwardly choose asubstrate material appropriate to a particular application.

In preferred embodiments, the small size of semiconductor die 300,particularly of an unpackaged semiconductor die 300, and itsabovementioned relatively low operating current and temperature, obviatethe need for a relatively high thermal conductivity substrate as isconventionally used, for example a ceramic substrate (such as Al₂O₃, AlNor the like) or metal-core printed circuit board (MCPCB) or a discreteor integrated heat sink (i.e., a highly thermally conductive fixture(comprising, for example, metal or ceramic materials) such as a plate orblock, which may have projections such as fins to conduct heat away andinto the surrounding ambient) to be in thermal communication withsemiconductor die 300. Rather, substrate 400 itself (as well as, e.g.,the adhesive, the traces, and even the surrounding ambient itself)provides adequate conduction of heat away from semiconductor die 300during operation.

In various preferred embodiments, one or more of the semiconductor dies300 on the substrate 400 are light-emitting devices such as LEDs and/orlasers. Conventional light-emitting assemblies are designed to maximizethe amount of light emitted per area. Such designs, which involveincreasing the amount of light emitted by each individual device,necessarily result in an increase in the amount of heat generated byeach device and thus typically require a low-thermal-resistance pathwayfrom the device (e.g., the LED junction) to ambient. Theselight-emitting assemblies may minimize the thermal resistance along thethermal pathway between the semiconductor die (e.g., a LED and thejunction of the LED) and the surrounding ambient via the use ofexpensive materials and/or complicated thermal-management schemes, e.g.,high-thermal-conductivity ceramics, thermal contact pads, metal-corecircuit boards, large heat sinks, and even active cooling devices suchas fans. Such devices frequently have thermal resistances of less than2.5° C./Watt (° C./W), or even less than 1° C./W.

For example the Cree XM-L packaged LED, which is representative ofhigh-brightness packaged LEDs, has a thermal resistance from thejunction to the solder point of 2.5° C./W. The Cree thermal managementguide CLD-AP05 REV 2 states that with good design, the thermalresistance from the solder point to the heat sink can be minimized toless than 1° C./W. The thermal resistance from the heat sink to ambientfor a given allowed junction temperature can then be calculated asfollows:R _(th.hs-a)=(T _(jmax) −T _(a) −R _(th.j-s) ×I×V−R _(th.s-hs)×I×V)/(I×V)where R_(th.hs-a) is the thermal resistance from heat sink to ambient,T_(jmax) is the maximum junction temperature, T_(a) is the ambienttemperature, R_(th.j-s) is the thermal resistance from the junction tothe solder point, I is the LED current, V is the LED voltage andR_(th.s-hs) is the thermal resistance from the solder point to the heatsink. If we allow T_(a) to be 55° C., and state that T_(jmax)=150° C.(from the Xm-L spec sheet), and we operate the LED at 1 A and 6 V, theLED power is 6 watts. The required heat sink must then have aR_(th.hs-a) of 12° C./W. Thus the total thermal resistance from junctionto ambient is 2.5+1+12=15.5° C./W. LEDs that emit relatively smalleramounts of light use packages with relatively higher thermal resistance.For example, parts designed to operate at about 20 mA typically have athermal resistance in the range of about 300° C./W.

In contrast, embodiments of the invention feature a high thermalresistance along the pathway from the semiconductor die 300 to thesurrounding ambient. This high thermal resistance may apply to eachindividual component along the pathway, e.g., the substrate 400, theadhesive 510, the traces 410, etc., and/or may apply collectively to theentire pathway. Specifically, the thermal resistance along the pathwayand/or of one or more of the components along the pathway may be greaterthan approximately greater than approximately 500° C./W, greater thanapproximately 1000° C./W, or even greater than approximately 2000° C./W.

For example, in one embodiment the thermal resistance from the p-njunction of the LED 300 to the adjacent trace 400 (in this example,silver) over substrate 400 (in this example, 5 mil thick PET) wasmeasured to be approximately 1800-2000° C./W. Part of the heat isdissipated by trace 400 and part of it flows through and is radiated outthe back of the substrate 400. PET film has a thermal resistance in therange of 8-18° C.-cm²/W. The die size used in this example was is 250μm×600 μm. If we assume that the area through which the heat flows is 1mm on a side, the area is 1 mm² and thus the thermal resistance of thePET is 13° C.-cm²/W (average thermal resistance) divided by the area(0.01 cm²) or 1300° C./W. Using an area of 1 mm² is overly conservativegiven the small size of the die and the fact that the PET sheet is only5 mil thick. Using a trapezoidal approximation, with the heat radiatingat a 50° angle, and taking the area as the average of the die size andthe projection on the back surface of the PET gives an area of 0.005cm². Using this area gives a thermal resistance of about 2600° C./W.Thus, in this example, the thermal resistance is at least 2000° C./W,and for the portion of the heat removed through the PET, at least 4500°C./W.

Based on these calculations, embodiments of the present invention have athermal resistance to ambient that is at least 100 times larger thanthat of conventional high-brightness LEDs. Furthermore, this can beachieved in certain embodiments with relatively low junctiontemperatures, e.g., below 100° C. In some embodiments, whensemiconductor die 300 comprises a p-n junction, the distance between thep-n junction and the surface of substrate 400 over which trace 410 isformed may be less than 100 μm, or less than 50 μm or less than 30 μm.In some embodiments, when semiconductor die 300 comprises a LED, thedistance between layer 350 (see FIG. 3B) and the surface of substrate400 over which trace 410 is formed may be less than 100 μm, or less than50 μm or less than 30 μm. In some embodiments, when semiconductor die300 comprises a device other than a LED, the distance between the heatgenerating region of semiconductor die 300 and the surface of substrate400 over which trace 410 is formed may be less than 100 μm, or less than50 μm or less than 30 μm. In some embodiments a shorter distance betweenthe p-n junction or the heat-generating region of semiconductor die 300and the surface of substrate 400 over which trace 410 is formed may beemployed in order to reduce the thermal resistance between the p-njunction (or the heat-generating region of semiconductor die 300) andthe ambient.

Embodiments of the present invention involve lighting assembliescomprising light-emitting semiconductor die attached to yieldingsubstrates using adhesives. Such assemblies comprise an array oflight-emitting elements disposed over substrate 400. In someembodiments, the light-emitting elements are disposed over substrate 400in a two-dimensional array with a pitch in the range of about 3 mm toabout 30 mm. For embodiments employing light-emitting semiconductor die300, the overall lighting assembly or module may produce at least 100lumens, at least 1000 lumens, or even at least 3000 lumens, and/or mayhave a density of semiconductor die 300 greater than approximately 0.25die/cm² of area over which the semiconductor die 300 are disposed. Suchlight-emitting systems may feature semiconductor die 300 having junctiontemperatures less than 100° C., or even less than 80° C. And, the heatdensity of such systems may be less than 0.01 W/cm² of area over whichthe semiconductor die 300 are disposed. Furthermore, the heat densitygenerated by systems in accordance with embodiments of the invention maybe less than approximately 0.01 W/cm², or even less than approximately0.005 W/cm², whereas conventional light-emitting devices typically haveheat densities greater than approximately 0.3 W/cm², or even greaterthan approximately 0.5 W/cm².

In one conventional approach, for example, a lighting assembly has oneLED and the area is the area of the printed circuit board (PCB) for thatLED. A similar definition may be used for lighting assemblies withmultiple LEDs, that is, the area is the PCB area over which the LEDs aredisposed. Based on this, a comparison between the present invention andthe prior art can be made using a 2′×2′ troffer which is conventionallyilluminated with fluorescent lamps. The prior-art approach to replacingthe fluorescent lamps with LEDs is to use a PCB that is significantlysmaller than the entire light-emitting area in combination with opticalcomponents to spread out the light. This may be accomplished by edgelighting an optical element, in which case the PCB for the LEDs may havedimensions on the order of 0.25″ by 24″ long, for an area of 6 in², or38.7 cm². In the case of LED-based fluorescent replacement lamps (alighting structure with a similar form factor to a fluorescent lamp, butthat produces light using LEDs), the PCB may be on the order of 24″ by1″, for an area of 24 in², or about 155 cm². These dimensions areassumed for what would be required for a 2′×2′ troffer. Assuming twoboards, the board area is no more than about 500 cm². This value is onthe high side for tube replacements and much larger than for the edgelighting approach. For embodiments of the present invention the area isthat of the entire 2′×2′ troffer, which is 576 in², or 3716 cm², largerby a factor of at least 7. Assuming that the LEDs in these luminaireshave an efficiency of 100 lm/W, before taking into account the powersupply efficiency, generation of 3500 lumens, which is a standardluminous flux for a 2′×2′ troffer, will require 35 watts. If the LEDsare 50% efficient, then about 17 watts of heat is generated. In theconventional case, the heat density is greater than 0.034 W/cm² for thetube replacement and about five times larger for the edge lightingapproach, while embodiments of the present invention may achieve a heatdensity on the order of 0.0045 W/cm²—almost a factor of 10 less thanthat for the conventional approach. The much smaller heat densityachievable using the present invention, relative to the heat density ofthe prior art, permits operation of lighting assemblies or moduleswithout additional heat sinking.

FIG. 4 depicts an exemplary substrate 400 for use in various embodimentsof the present invention. Substrate 400 is preferably yielding, i.e.,flexible and/or deformable, and may be flexible or rigid to permitelectrical coupling between contacts on the semiconductor die and traceson the substrate using pressure-activated adhesive—even in embodimentswhere the contacts on the semiconductor die are non-planar—withoutdamaging the semiconductor die. This may be achieved, for example, bythe substrate flexing as shown in FIG. 5A or deforming as shown in FIG.5B. Such deformation may be elastic (returning to the original shapeafter the load is removed) or plastic deformation (maintaining permanentdeformation after the load is removed) or a combination of elastic andplastic deformation. In various embodiments, the substrate may both flexand deform. In some embodiments, substrate 400 is flexible and has aradius of curvature of about 1 m or less, or about 0.5 m or less, oreven about 0.1 m or less. In some embodiments, substrate 400 has aYoung's Modulus less than about 100 N/m², less than about 50 N/m², oreven less than about 10 N/m². In some embodiments, substrate 400 has aShore A hardness value less than about 100; a Shore D hardness less thanabout 100; and/or a Rockwell hardness less than about 150.

Substrate 400 may include or consist essentially of a semicrystalline oramorphous material, e.g., polyethylene naphthalate (PEN), polyethyleneterephthalate (PET), polycarbonate, polyethersulfone, polyester,polyimide, polyethylene, and/or paper. Substrate 400 may comprisemultiple layers, e.g., a deformable layer over a rigid layer, forexample, a semicrystalline or amorphous material, e.g., PEN, PET,polycarbonate, polyethersulfone, polyester, polyimide, polyethylene,and/or paper formed over a rigid substrate for example comprising,acrylic, aluminum, steel and the like. Depending upon the desiredapplication for which embodiments of the invention are utilized,substrate 400 may be substantially optically transparent, translucent,or opaque. For example, substrate 400 may exhibit a transmittance or areflectivity greater than 80% for optical wavelengths ranging betweenapproximately 400 nm and approximately 600 nm. In some embodimentssubstrate 400 may exhibit a transmittance or a reflectivity of greaterthan 80% for one or more wavelengths emitted by semiconductor die 300.Substrate 400 may also be substantially insulating, and may have anelectrical resistivity greater than approximately 100 ohm-cm, greaterthan approximately 1×10⁶ ohm-cm, or even greater than approximately1×10¹⁰ ohm-cm.

As shown in FIG. 4, at least two conductive traces 410 are disposed onthe substrate 400 to provide electrical connectivity to a device or dieconnected to the traces. The traces 410 are spaced apart, defining a gap420 therebetween that may be sized based on the size of the device ordie and contact spacings on the device or die to be connected to thetraces. For example, the gap 420 may range between approximately 25 μmand approximately 1000 μm. The traces 410 preferably include or consistessentially of one or more conductive materials, e.g., a metal or metalalloy, carbon, etc. Traces 410 may be formed via conventionaldeposition, photolithography, and etching processes, plating processes,or may be formed using a variety of printing processes. For example,traces 410 may be formed via screen printing, flexographic printing, inkjet printing, and/or gravure printing. The traces 410 may include orconsist essentially of a conductive ink, which may include one or moreelements such as silver, gold, aluminum, chromium, copper, and/orcarbon. As mentioned above, preferred embodiments of the invention donot utilize stud bumps or similar conductive projections over traces410; therefore, the distance between substrate 400 and a device bondedto substrate 400 is at least in part defined by the thickness of traces410 (which are typically equal to each other). This thickness of traces410 is preferably less than approximately 10 μm, and even morepreferably less than approximately 5 μm. While the thickness of one ormore of the traces 410 may vary, the thickness is generallysubstantially uniform along the length of the trace to simplifyprocessing. However this is not a limitation of the present inventionand in other embodiments the trace thickness or material may vary onsubstrate 400.

Referring to FIG. 5A, in various embodiments semiconductor die 300 isbonded (i.e., attached) to substrate 400. In order to enable electricalconnectivity to semiconductor die 300, contacts 370, 380 are typicallyadhered to (e.g., directly to) and in electrical contact with traces410. As shown in FIG. 5A with a yielding substrate 400, strong reliablebonds between the traces and the contacts are achieved by flexing (i.e.,bending or deforming) at least in a region 500 between the traces 410.Substrate 400 may flex such that the distances between each of contacts370, 380 and its corresponding trace 410 (to which it is adhered) areapproximately equal. In preferred embodiments, the contacts 370, 380 areadhered to traces 410 via a pressure-activated adhesive 510. Forexample, adhesive 510 may include or consist essentially of apressure-activated ACA, and thus contacts 370, 380 may be electricallyconnected to traces 410 via conductive structures such as particleswithin the ACA, while the contacts 370, 380 are electrically insulatedfrom each other (as are the traces 410).

In another embodiment, shown in FIG. 5B, electrical conductivity isenabled by deformation of substrate 401. In this embodiment, a portionof semiconductor die 300 or contacts 370, 380 deforms a portion ofsubstrate 401 in region 501 and by such deformation electricalconductivity between traces 410 and contacts 370, 380 is enabled. InFIG. 5B, substrate 401 is shown as deforming only in the surface region,with no deformation of the face of substrate 401 opposite the face overwhich conductive traces 410 are formed. This is not necessary to thepresent invention, however, and in other embodiments, deformation mayoccur on both faces of substrate 401. Indeed, the substrate may bothflex and deform, combining the behavior illustrated in FIGS. 5A and 5B.

If substrate 400 is too soft, pressure applied across semiconductor die300 and substrate 400 may result in deformation of substrate 400 withoutsufficient force being applied to the ACA to establish electricalconnection between traces 410 and contacts 370, 380. If substrate 400 istoo hard, on the other hand, pressure applied across semiconductor die300 and substrate 400 may result in fracture or breaking ofsemiconductor die 300 before the ACA is able establish electricalconnection between traces 410 and contacts 370, 380. Thus the requiredlevel of deformability for substrate 400 may also depend on themechanical properties of semiconductor die 300; tougher semiconductordie 300 may permit use of relatively less deformable substrate 400.Conversely, more fragile semiconductor die 300 may require use of arelatively more deformable substrate 400. Those of skill in the art maystraightforwardly determine, without undue experimentation, theappropriate degree of substrate hardness for a particular semiconductordie. In some applications, the toughness of semiconductor die may bevaried by changing its thickness or the materials from which it isfabricated.

During the bonding of semiconductor die 300 to substrate 400, adhesive510 may be dispensed in substantially liquid form, i.e., as a paste or agel, as opposed to a solid such as a tape. The adhesive 510 may bedispensed over portions of semiconductor die 300 (e.g., at leastportions of contacts 370, 380) or substrate 400 (e.g., at least portionsof traces 410) or both. Contacts 370, 380 are then brought into physicalproximity (or contact) with and adhered to traces 410 via application ofpressure to semiconductor die 300, substrate 400, or both. Becauseadhesive 510 in some embodiments is an ACA, perfect alignment betweencontacts 370,380 and traces 410 is not necessary, thus simplifying theprocess. When using an ACA, perfect alignment is not required becauseconduction occurs only in the vertical direction between contacts 370,380 and traces 410, and not laterally between contacts 370, 380 orbetween traces 410. In one embodiment, semiconductor die 300 andsubstrate 400 are compressed between a substantially rigid surface and asubstantially compliant surface, thereby enabling the flexure ordeformation or both of substrate 400 depicted in FIGS. 5A and 5B and theresulting electrically conductive and reliable bond to semiconductor die300 notwithstanding the nonplanarity of surface 360 and/or thenon-coplanarity between contacts 370, 380.

After or during the compression of semiconductor die 300 and substrate400 (and, in preferred embodiments, pressure-induced activation ofadhesive 510), adhesive 510 is cured by, e.g., application energy, forexample heat and/or ultraviolet light. For example, adhesive 510 may becured by heating to a temperature ranging from approximately 80° C. toapproximately 150° C., e.g., approximately 125° C., for a period of timeranging from approximately several seconds to 1 minute to approximately30 minutes, e.g., approximately 10 minutes, depending on the propertiesof the adhesive.

In another embodiment, the adhesive 510 comprises an isotropicallyconductive adhesive in regions 520 between contacts 370, 380 and theirrespective traces 410. In such embodiments, in a region 530 between thetraces 410 and between contacts 370, 380, insulation may be maintainedvia absence of adhesive 510 or via the presence of a second,non-conductive adhesive. Adhesive 510 preferably features a polymericmatrix, rather than a fully metallic one that might result inundesirable electrical shorting between contacts 370, 380 and/or betweentraces 410. In some embodiments adhesive 510 may be reflective to atleast some or all wavelengths of light emitted by semiconductor die 300.

FIG. 6A depicts an electronic device 600 featuring an array ofsemiconductor die 300 adhered between conductive traces 410 as describedabove. As shown, electronic device 600 includes three serially-connectedstrings 610 of semiconductor dies 300. Electronic device 600 alsoincludes circuitry 620 electrically connected to one or more of thestrings 610. The circuitry 620 may include or consist essentially ofportions of (in the case, for example, of a distributed powersupply/driver) or portions of or substantially all of drive circuitry,sensors, control circuitry, dimming circuitry, and or power-supplycircuitry or the like, and may also be adhered (e.g., via an adhesive)or otherwise attached to substrate 400. Circuitry 620 may even bedisposed on a circuit board (e.g., a printed circuit board) that itselfmay be mechanically and/or electrically attached to substrate 400. Inother embodiments circuitry 620 is separate from substrate 400. WhileFIG. 6A depicts the semiconductor die 300 serially connected in strings610, and strings 610 connected or connectable in parallel (see alsoFIGS. 7A and 7B), other die-interconnection schemes are possible andwithin the scope of the invention.

Furthermore, one or more semiconductor die 300 may be bonded to traces410 on the back side of substrate 100 in a similar or different fashionto that depicted in FIG. 6A, and/or multiple substrates 400 havingsemiconductor dies 300 and traces 410 thereon may be stacked to formmulti-layer devices. In these embodiments, with die on the front andback of substrate 400 or multiple substrates 400, the die within as wellas on each layer may all be the same or may be different; for example,semiconductor die 300 on different layers may emit at differentwavelengths. In devices having semiconductor die 300 on the substrateback side or disposed in multiple layers, each layer may have its owndedicated circuitry 620, or all or part of circuitry 620 may be sharedamong layers and/or groups of semiconductor die 300. The circuitry 620may include or consist essentially all or portions of any of theembodiments described in U.S. patent application Ser. No. 12/982,758,filed on Dec. 30, 2010, the entire disclosure of which is incorporatedby reference herein. In some embodiments, semiconductor die and/orcircuit elements on the back or front, or on multiple layers ofsubstrate 400 may be electrically coupled together.

Since electronic device 600 may be based on a yielding substrate 400, itmay be formed in a roll-to-roll process, in which a sheet of theyielding substrate material travels through different processingstations. Such roll-to-roll processing may, for example, include theformation of traces 410, dispensing of the adhesive 510, and theplacement of semiconductor dies 300, as well as for the bonding of anyadditional substrates and/or formation of one or more phosphor materials(as detailed below). In addition, electronic device 600 may also includeother passive and/or active electronic devices attached to substrate400, including, e.g., sensors, antennas, resistors, inductors,capacitors, thin-film batteries, transistors and/or integrated circuits.Such other passive and/or active electronic devices may be electricallycoupled to traces or semiconductor dies 300 with adhesive 510 or byother means.

Furthermore, as shown in FIG. 6B, two or more semiconductor dies 300 maybe connected in parallel to the same traces 410 (i.e., within the samegap 420 between traces), thus providing enhanced functionality and/orredundancy in the event of failure of a single semiconductor die 300. Ina preferred embodiment, each of the semiconductor dies 300 adheredacross the same gap 420 is configured not only to operate in parallelwith the others (e.g., at substantially the same drive current), butalso to operate without overheating or damage at a drive currentcorresponding to the cumulative drive current operating all of thesemiconductor dies 300 disposed within a single gap. Thus, in the eventof failure of one or more of the semiconductor dies 300 adhered acrossthe gap 420, the remaining one or more semiconductor dies 300 willcontinue to operate at a higher drive current. For example, forsemiconductor dies 300 including or consisting essentially oflight-emitting devices such as LEDs or lasers, the failure of a deviceconnected in parallel to one or more other devices across the same gapresults in the other device(s) operating at a higher current and thusproducing light of increased intensity, thereby compensating for thefailure of the failed device.

FIG. 6B also illustrates two of the different adhesion schemes describedabove. One of the semiconductor dies 300 is adhered to the traces 410via adhesive 510 only at the ends of the die, while between the endswithin the gap between traces, a second adhesive 630 (which ispreferably non-conductive) adheres the middle portion of thesemiconductor die 300 to substrate 400. In some embodiments the secondadhesive 630 is non-conductive and prevents shorting between the twoportions of conductive adhesive 510 and/or between traces 410 and/orbetween the two contacts of die 300. As shown, the other semiconductordie 300 is adhered between the traces 410 with adhesive 510 contactingthe entirety of the bottom surface of semiconductor die 300. Asdescribed above, adhesive 510 is preferably a pressure-activated ACAthat permits electrical conduction only in the vertical direction (outof the plane of the page in FIG. 6B) but insulates the traces 410 fromeach other. In other embodiments, one or more semiconductor dies 300 areadhered between traces 410 within the same gap 420, but there issufficient “real estate” within the gap 420 (including portions of thetraces 410) to adhere at least one additional semiconductor die 300within the gap 420. In such embodiments, if the one or moresemiconductor dies 300 initially adhered within the gap 420 fail, thenone or more semiconductor dies 300 (substantially identical to ordifferent from any of the initial semiconductor dies 300) may be adheredwithin the gap 420 in a “rework” process. For example, referring to FIG.6B, only one of the depicted semiconductor dies 300 may be initiallyadhered to the traces 410, and the other semiconductor die 300 may beadhered later, e.g., after failure of the initial die.

FIGS. 7A and 7B schematically depict two different layouts of electricaltraces 410 that may be utilized in electronic devices in accordance withvarious embodiments of the invention. Much as in FIG. 6A, FIGS. 7A and7B depict parallel strings 610 of traces 410 configured to interconnectmultiple semiconductor dies 300 in series (while the gaps 702representing bonding locations for the semiconductor dies 300 are shownin FIG. 7A they are omitted in FIG. 7B for clarity). In FIG. 7A, eachstring 610 has a contact 700 at one end and a contact 710 at the other.In various embodiments, contact 700 is a “drive” contact for applyingoperating current or voltage to the semiconductor dies 300, whilecontact 710 is a “common” or ground contact. In FIG. 7B, each string 610extends across substrate 400 and turns back to extend back to a pointnear its starting point, enabling both contacts 700, 710 to be placed onone side of substrate 400. As also shown in FIG. 7B, either or both ofcontacts 700, 710 for multiple strings 610 may be connected togetherinto a shared contact (as shown of contacts 710 in FIG. 7B); suchschemes may simplify layout and interconnection of the semiconductordies 300 and/or strings 610. While the layouts depicted in FIGS. 7A and7B position the semiconductor dies 300 in a square or rectangular grid,the semiconductor dies 300 may be arranged in other ways. Likewise, thetraces 410 may be substantially straight, as shown, or may be curved,jagged, non-parallel, or be arranged in other ways.

In embodiments in which one or more of the semiconductor dies 300 is alight-emitting device such as a LED or a laser, a phosphor material maybe incorporated to shift the wavelength of at least a portion of thelight emitted by the die to another desired wavelength (which is thenemitted from the larger device alone or color-mixed with another portionof the original light emitted by the die). As used herein, “phosphor”refers to any material that shifts the wavelength of light irradiatingit and/or that is luminescent, fluorescent, and/or phosphorescent.Phosphors comprise powders or particles and in such case may be mixed inbinders, e.g., silicone. As used herein, phosphor may comprise thepowder or particles or to the powder or particles plus binder. FIGS.8A-8D depict an exemplary procedure for integrating phosphors with thesemiconductor dies 300 adhered to a yielding substrate 400. FIG. 8Adepicts a cross-sectional view of two semiconductor dies 300 adhered toa substrate 400, each across a gap 420 between two conductive traces 410(flexure and/or deformation of substrate 400, any non-planarity of thesemiconductor dies 300, and the adhesive 510 have been omitted from thefigures for clarity). A substrate 800 having an opening 810 (whichpreferably extends through the entire thickness of substrate 800)corresponding to one or more of the semiconductor dies 300 on substrate400 is provided (FIG. 8A) and bonded to substrate 400 such that one ormore of the semiconductor dies 300 is positioned within an opening 810(FIG. 8B). The substrate 800 may be yielding or substantially rigid, andmay include or consist essentially of materials such as polyethylenenaphthalate, polyethylene terephthalate, polycarbonate,polyethersulfone, polyester, polyimide, polyethylene, and/or paper.

As shown in FIG. 8C, the openings 810 in substrate 800 form “wells”around semiconductor dies 300. After substrate 800 is bonded tosubstrate 400, a phosphor 820 is provided within one or more of theopenings 810 such that phosphor 820 is disposed over and at leastpartially surrounds (e.g., is on one or more sides of, but notnecessarily in contact with) the semiconductor die 300 in the opening810. As shown, the phosphor 820 may substantially fill the opening 810,and may be in contact with semiconductor die 300. In other embodiments,an optically transparent material 830 (e.g., silicone or epoxy) isprovided within and partially fills one or more of the openings 810before the phosphor 820 is disposed within (and may substantially fillthe remainder of) the opening 810 (see the right opening 810 in FIG.8C). This “remote phosphor” arrangement positions the phosphor 820 at adistance from the semiconductor die 300, which may prevent the operationof semiconductor die 300 from deleteriously heating the phosphor 820,thus extending its lifespan and/or improving its efficiency. In someembodiments, openings 810 are not completely filled with phosphor 820 orclear material 830, whereas in other embodiments, openings 810 areover-filled with phosphor 820 or clear material 830. Openings 810 maynot have any phosphor 820 and/or may not have any clear material 830. Insome embodiments, multiple optically transparent and phosphor materialsare formed in layers or other configurations.

As shown in FIG. 8D, a protective film 840 may be optionally placed oversubstantially all of substrate 800 and openings 810, or at least overopenings 810 having phosphor 820 and/or clear material 830 therewithin,thereby protecting the phosphor 820 and/or the semiconductor dies 300from the surrounding ambient, moisture, and the like. The film 840 maybe transparent, or may be reflective such that the light emitted bysemiconductor die 300 and shifted by the phosphor 820 is reflectedtoward and emitted from the substrate 400. In some embodiments, multipledifferent phosphors 820 are disposed in one or more of the openingsabove the semiconductor die 300. That is, one well 810 may have morethan one type of phosphor 820 and/or clear material 830. Different wells810 may, for example, have different phosphors 820 and/or differentclear materials 830. In one embodiment, different semiconductor die 300(e.g., emitting at different wavelengths) may be associated with thesame or different phosphors 820 and/or clear materials 830.

In some embodiments, the sidewalls of the openings 810 are notsubstantially perpendicular to the surface of substrate 400 (as shown inFIG. 8C), but are sloped or otherwise shaped and/or patterned tofacilitate the out-coupling of light from the semiconductor die 300and/or out-coupling of light from phosphor 820. The sidewalls of theopenings 810 may even be reflective to light emitted by semiconductordie 300 or the light from phosphor 820 (e.g., coated with a reflectivematerial). An optical element such as a lens or diffuser may bepositioned above the semiconductor die 300 and/or the phosphor 820. Well810 may have any shape appropriate to the application, e.g., round,rectangular, hexagonal shape or any arbitrary shape. Different wells 810may, in fact, have different shapes.

The phosphor 820 may include or consist essentially of, e.g., one ormore silicates, nitrides, quantum dots, or other light-conversionmaterials, and may be suspended in an optically transparent binder(e.g., silicone or epoxy). Semiconductor dies 300 for use with one ormore phosphors 820 may emit substantially blue or ultraviolet light, andthe use of the phosphor(s) 820 may result in aggregate light that issubstantially white, and which may have a correlated color temperature(CCT) ranging from approximately 2000 K to approximately 7000 K.Examples of such die include those comprising GaN, InN, AlN and variousalloys of these binary compounds.

FIGS. 9A-9D illustrates another embodiment of the invention featuring aremote phosphor for wavelength conversion of at least a portion of thelight emitted from a semiconductor die 300. FIG. 9A depicts a portion ofan electronic device 900 similar to electronic device 600 including asemiconductor die 300 adhered to a substrate 400 across a gap 420between two conductive traces 410 (flexure and/or deformation ofsubstrate 400, any non-planarity of the semiconductor die 300, and theadhesive 510 have been omitted from the figures for clarity). As shownin FIG. 9B, an optically transparent material 910, which may be designedto provide good optical coupling with the index of refraction ofsemiconductor die 300, is formed on semiconductor die 300. In someembodiments, the index of refraction of transparent material 910 isbetween about 1.0 and about 1.65 (e.g., ranging from 1.4 to 1.57). Whiletransparent material 910 is illustrated as having a hemispherical shape,this is not necessary to the present invention and transparent material910 may have virtually any shape. In some embodiments, transparentmaterial 910 is patterned with a surface profile or texture in order toincrease the surface area of phosphor 920 and/or reflective layer 930.

A phosphor 920 (or multiple layers of different phosphors) is formedover the material 910, as shown in FIG. 9C. Phosphor 920 may be any oneor more of the materials described above with reference to phosphor 820,and material 910 physically separates phosphor 920 from thesemiconductor die 300. While various embodiments of the inventionutilize the structure of FIG. 9C to enable emission of color-mixed orconverted light through phosphor 920 into the surrounding ambient, FIG.9D depicts a preferred embodiment in which a reflective layer 930 isformed over the phosphor 920. In the embodiment of FIG. 9D, theconverted or color-mixed light reflects from reflective layer 930 afterinteracting with phosphor 920 and is emitted from device 900 throughsubstrate 400. Reflective layer may be, for example, a highly specularor diffuse reflector. In one embodiment reflective layer 930 is a metal,e.g., aluminum, silver, gold or the like. Reflective layer 930 may be awhite reflector, e.g., MCPET. Various phosphor deposition techniques maybe employed, including those described in Donofrio, R. L., “PhosphorScreening,” SID Sixth International Conference on Advanced Displays(1997), pp. 89-95, the disclosure of which is hereby incorporated byreference. In a preferred embodiment, highly reflective layer 360comprises an electrically conductive material such that electrophoreticdeposition may be employed to apply a conformal coating of phosphor 920with uniform thickness.

In one embodiment, die 300 emits blue light 940 (FIG. 9D). In operation,blue light 940 emitted by LED 300 intersects with phosphor layer 920.Some of the light is absorbed by phosphor particles in phosphor layers920, whereupon it is re-emitted at a different wavelength. The remainderof the blue light 940 is specularly reflected from the highly reflectivesurface 930. Some of this blue light is again absorbed by phosphorparticles in phosphor layer 920 and re-emitted. The re-emitted lightfrom the phosphor is emitted isotropically by the phosphor particles.Half of the light will be specularly reflected from the highlyreflective surface 930, such that substantially all of the light will beemitted into transparent material 910. Depending on the angle of theemitted light, it will either intersect the exterior surface of phosphorlayer 930 (shown as ray 950) or exit the cavity via transparentsubstrate 400 (shown as ray 960).

FIG. 10 depicts another embodiment of device 900 in which the phosphor920 is formed directly on substrate 400 (the dashed portions of phosphor920 represent the portions disposed in front of and/or behind the traces410) rather than over material 910. In the embodiment of FIG. 10, lightemitted from semiconductor die 300 is reflected by reflective layer 930back toward substrate 400, and at least a portion of the light interactswith phosphor 920 such that the aggregate light emitted from substrate400 is the desired wavelength or mix of wavelengths (e.g., white light).In the embodiments of FIGS. 9D and 10, traces 410 may be relativelynarrow or substantially transparent in order to prevent undesiredblocking or back-reflection of light. For examples, transparent traces410 may include or consist essentially of indium tin oxide, indium zincoxide, aluminum zinc oxide, carbon nanotubes, graphene, and/orconductive polymers such as poly(3,4-ethylenedioxythiophene doped withpoly(styrene sulfonate).

As shown in FIG. 11, the material 910, phosphor 920, and reflectivelayer 930 may be formed within a depression, cavity, or other opening1100 in a substrate 1110, which is then bonded to substrate 400 suchthat these layers are disposed over semiconductor die 300 similarly tothe embodiment of FIG. 9D. In such embodiments, material 910 may even beomitted (i.e., may be air or vacuum). Although not explicitly depicted,reflective layer 930 may be formed in the depression of substrate 1110and phosphor 920 may be formed proximate semiconductor die 300 onsubstrate 400 in order to form a structure resembling that of FIG. 10.Substrate 1110 may be yielding or substantially rigid, and may even beoptically translucent or opaque, as light from the semiconductor die 300is emitted through substrate 400. Cavity 1100 is preferablysubstantially hemispherical in shape, but may also take other shapes,e.g., ellipsoidal, paraboloidal, hyperboloidal, or pyramidal (with threeor more substantially planar sides). In some embodiments, cavity 1100 ispatterned with a surface profile or texture in order to increase thesurface area of phosphor 920 and/or reflective layer 930. In oneembodiment, the surface of substrate 400 facing transparent material 910is treated with an antireflection coating to minimize reflections fromsaid surface.

Embodiments of the present invention offer numerous advantages,including, without limitation, the following. First, the transparentmaterial 910, e.g., air, epoxy or silicone, thermally insulates thephosphor layers 370 and 380 from the LED die, alleviating the risk ofthermal quenching. Second, dichroic mirrors are not required, greatlyimproving manufacturability and reducing cost. Instead, the mirroredsurface 360 reflects any light emitted by the phosphor particlesincident on it back through phosphor layer 920 to transparent substrate400. Third, the hemispherical shape of the remote phosphor shell doublesthe surface area of the phosphor layer 920 exposed to the blue lightemitted by the LED die 300. This effectively doubles the luminousexitance of the circular opening in substrate 400 defined by transparentmaterial 910. (In other words, it effectively doubles the photometricbrightness of the opening as viewed from all angles through transparentsubstrate 400, due to the increased phosphor layer surface area.)

Fourth, it has been shown (see, e.g., Yamada, K., Y. Imai, and K. Ishi,“Optical Simulation of Light Source Devices Composed of Blue LEDs andYAG Phosphor,” Journal of Light & Visual Environment 27(2):70-74 (2003)(hereafter “Yamada et al.”)) that YAG:Ce phosphor saturates at aconcentration of approximately 20% by weight in transmission mode, butsaturates at a concentration of 50% to 60% by weight in reflection mode.The phosphor layer 920 may therefore have a higher concentration and soprovide increased conversion efficiency. As demonstrated by Yamada etal., increased conversion efficiencies of 50% are possible with YAG:Cephosphor materials. Fifth, it has also been shown (see, e.g., Yamada etal.) that the chromaticity of light generated by blue InGaN LEDs andYAG:Ce phosphor exhibits considerably less variation with phosphorconcentration in reflection mode versus transmission mode. Hence, thepresent invention may provide relaxed manufacturing tolerances for thethickness and uniformity of the phosphor layer 920.

Sixth, the mirrored surface 930 reflects incident light regardless ofits angle of incidence and regardless of whether it is scattered bluelight from the LED die 300 or emitted light from the phosphor layer 920.Seventh, the radiation emitted from the phosphor layer surface has aLambertian distribution. This includes both the reflected blue light andthe phosphor-emitted light. In accordance with radiative flux transfertheory and view factor geometry, exactly one half of this light will beincident upon substrate 400, while the other half will be incident onthe phosphor layer. Depending on its wavelength, this light may furtherexcite the phosphor particles, thereby providing a form of positiveoptical feedback that further improves the down-conversion efficiency ofthe phosphors. (For example, the excitation and emission spectra ofYAG:Ce overlap in the region of approximately 475 nm to 525 nm, so thatemitted light within this region self-excites the phosphor rather thanbeing absorbed.)

The shape of the transparent material 910 is nominally a hemisphere. Ifit is shallower, the surface area of the phosphor layer is reduced,which tends to reduce the luminous exitance of the circular opening insubstrate 400 defined by the transparent material 910. However, thelight from the LED die 300 will not be at normal incidence to thephosphor surface at the periphery of the cavity. This will tend toincrease specular reflections from the phosphor layer in accordance withthe Fresnel equations, which may enhance the luminous exitance of thecircular opening. If transparent material 910 is deeper, more than onehalf of the light will undergo multiple reflections within the cavity,which will tend to reduce the luminous exitance. However, this may beoffset by self-excitation of the phosphor layers. In practice, then, adeeper or shallower shape than a hemispherical shape for transparentmaterial 910 may be optimal, depending on the bidirectional reflectancedistribution function (BRDF) of the exposed phosphor layer surface andthe optical gain provided by self-excitation of the phosphor from itsown emission. The optimal cavity shape may therefore be ellipsoidal,paraboloidal, or hyperboloidal. It may also be pyramidal, with three ormore planar sides.

The reflection (on average) of one-half of the light emitted by thephosphor particles by the mirrored surface 930 and scattered by phosphorlayer 920 will tend to homogenize the light emitted from the circularopening in substrate 400 defined by the transparent material 910. Thiswill, as a consequence, improve both intensity and color uniformity,further relaxing the manufacturing tolerances for the phosphor layerthickness and phosphor particle density.

FIG. 12A shows an example of an electronic module 1200 (e.g., a lightingmodule) according to various embodiments of the present invention. Themodule 1200 may have a generally planar shape with a relatively thinprofile. In its initial, or rest state, module 1200 may be flat, curvedin one direction, curved in two directions, or it may have a morecomplex curvature. The module 1200 may feature a substantially yieldingsubstrate 400 having an array of semiconductor dies 300 thereon (notshown in this figure). Semiconductor die 300 may be organized in aregular or random array on substrate 400. In the embodiment wheresemiconductor die 300 comprises a LED, the LED pitch (that is, thespacing between LEDs in the array) in the array may vary from about 2 mmto about 25 mm. In one embodiment, the LED pitch is determined bydividing the required total amount of light from LEDs for module 1200 bythe light emitted by one LED. It will be clear to one skilled in the artthat the LED pitch is a function of the amount of light emitted by oneLED. For example, the same amount of total light may be produced usingrelatively more LEDs emitting relatively less light but with arelatively smaller LED pitch as by relatively fewer LEDs emittingrelatively more light with a relatively larger LED pitch. In oneembodiment, the LED pitch is at least in part determined by the distancebetween the LED and any associated optics or diffusers (incorporated,for example, in plate 1240 or otherwise). In one embodiment, the LEDpitch is similar to or substantially the same as the distance betweenthe LED and an associated diffuser.

One or more circuit boards may be coupled to the substrate 400. Asshown, three circuit boards 1210, 1220, 1230 are attached to substrate400. The circuit boards 1210, 1220, 1230 may have a long, thinrectangular shape in order to be positioned at the edges of thesubstrate 400. Portions or all of the drive circuitry, for examplecurrent-source components, may be disposed on one or more of the circuitboards 1210, 1220, 1230, which may be yielding or substantially rigid.In an embodiment, one or more of the circuit boards 1210, 1220, 1230includes or consists essentially of a printed circuit board (PCB)attached to the substrate 400 with, for example, a connector, conductiveadhesive, anisotropic conductive adhesive or film or conductive epoxy orflexible connector to connect various components to individualsemiconductor dies 300. In one embodiment, circuit boards 1210, 1220,1230 are electrically coupled to substrate 400 with flexible connectors,permitting flexibility in the positioning of circuit boards 1210, 1220,1230 relative to substrate 400.

Optionally, a transparent plate 1240 may be located on top of thesubstrate 400. In an embodiment, the plate 1240 is patterned withlocalized deposits of phosphor 920, as shown in FIG. 12B, which alignwith light-emitting semiconductor dies 300 such that light emitted bythe dies 300 irradiates the various phosphor deposits. In a preferredembodiment, the combination of light from the semiconductor dies 300 andthe light emitted from the phosphor 920 produces white light with any ofa variety of correlated color temperatures (CCTs). In other embodiments,the phosphors 920 are formed over the semiconductor dies 300 as shown inFIG. 9A-9D, 10, or 11, or may even be formed as a substantiallycontinuous layer on a surface of plate 1240. FIG. 12B is a magnifiedview of a corner of the module 1200 shown in FIG. 12A. As shown, thecircuit board 1210 may act as a location stop for the transparent plate1240. Likewise, the other circuit boards 1220, 1230 may provide amechanical location reference for the plate 1240. FIG. 13 depicts anexploded view of the module 1200, showing the substrate 400, the circuitboards 1210, 1220, 1230, and the plate 1240 with deposits of phosphor920.

Various embodiments of the invention feature different physicalconfigurations. For example, the module 1200 may have one, two, four, ormore circuit boards. One or more of the circuit boards may notnecessarily extend the full length of an edge of the substrate 400,and/or two or more circuit boards may be affixed to the same edge of thesubstrate 400. The circuit board(s) may not lie flush with the edges ofthe substrate 400, but rather may overhang one or more edges or may bepositioned a distance away from an edge. A blank (i.e., opticallytransparent, without deposits of phosphor 920) piece of material may beadded as an additional locator for the plate 1240. The plate 1240 mayinclude optics such as lenses, waveguides, reflectors, diffractorsand/or diffusers.

Electronic modules 1200 may be fabricated by assembling differentsubstrates 400 with wire bonds, soldered jumper wires, flexibleconnectors, anisotropic conductive films or other means of electricalconnection to produce arrays of one or more tiles. An embodiment of sucha module 1400 is shown in FIG. 14, which as shown is fabricated frommultiple substrates 400. The substrates 400 may be mounted on a planarcarrier 1410, and may be bounded on one or more sides by circuit boards,e.g., circuit boards 1210, 1220, 1230. As described above for module1200, module 1400 may also feature a plate 1240 with areas of phosphor920 or optical elements aligning with the semiconductor dies 300 on thevarious substrates 400.

Any or all of the circuit boards 1210, 1220, 1230 and multiplesubstrates 400 may be mounted on a single large-area transparent carrier1410 with or without phosphors and or optical elements to form a thinpanel with a substantially constant luminance distribution, suitable fora wide range of uses, for example for general or architectural lightingapplications or as a backlight unit for LCD display panels. FIG. 15shows an exploded view of such an electronic module 1400, showing themultiple substrates 400, circuit boards 1210, 1220, 1230, and the plate1240 with regions of phosphor 920 and or optical elements. In someembodiments, any or all of the various substrates 400 in module 1400 maybe different from each other, e.g., support different numbers and/ortypes of semiconductor dies 300, phosphors and/or optical elements. Forexample, a different substrate 400′ may be used for the interiorpositions and yet another different substrate 400″ may be used for thecorner positions. The individual substrates 400 may be square,rectangular, hexagonal, triangular, L-shaped, or any other tessellatingor non-tessellating shape. In some embodiments, phosphor 920 is the sametype of phosphor at all locations, while in other embodiments differentphosphors may be used at different locations. Phosphor 920 may beintegrated in ways other than on plate 1240, for example, as shown inFIGS. 8-11. The shapes of modules 1200 are not critical, and thesemodules may be, for example, rectangular, square, hexagonal or any othershape to meet design, architectural or lighting needs.

FIG. 16A depicts an electronic module 1600 that includes a sensor 1610for detecting such features as room occupancy, ambient light, or otherenvironmental factors known to those skilled in the art. One or moresuch sensors 1610 may be included in the module 1600, and thus module1600 may detect more than one environmental factor. Feedback (i.e.,signals) from the sensor 1610 may be used to operate the module 1600,e.g., operating one or more semiconductor dies 300. For example,light-emitting semiconductor dies 300 may be turned on or off oroperated so as to dim the light emitting therefrom (either immediatelyor after a time delay). A drive circuit 1620 may be mounted withinmodule 1600 and may include a feedback system to enable the data fromthe sensor 1610 to operate the module 1600. Drive circuit 1620 mayinclude or consist essentially of a dimming circuit. The module 1600 mayalso include a support 1630 (including or consisting essentially of,e.g., plexiglass or another substantially rigid material), a supportframe 1640, and a substrate 400 having one or more semiconductor dies300 adhered thereto. A cover plate 1650 (including or consistingessentially of, e.g., plexiglass or another substantially transparent ortranslucent material) may also include additional optics.

In various embodiments, a light sensor 1610 may be incorporated intoeach of multiple modules 1600 functioning as a luminaire, such that thelight sensor 1610 samples the ambient that is substantially illuminatedby that luminaire. If the light intensity is larger than a certainthreshold level, the module(s) 1600 in the luminaire are dimmed to apoint where the sensed light intensity (i.e., the aggregate lightintensity from other sources in the ambient, e.g., sunlight, and themodule 1600 itself) is at the threshold value. In this manner, a new orretrofit unit incorporating one or more modules 1600 may providesubstantial energy savings through daylight harvesting, without the needto install an expensive central lighting control system. This is aparticular advantage when installing retrofit units, as it obviates theneed to install wiring required for a central control system in anexisting building or other installation.

In another embodiment, an occupancy sensor 1610 is incorporated into oneor more modules 1600 functioning as a luminaire. In a similar manner asdiscussed above, the occupancy sensor 1610 may sample the areailluminated by the luminaire, and if no occupant is present, dim or turnoff the luminaire. This may result in energy savings without a “pillarof light” situation, where only one light is on over an occupied area.Such modules 1600 may also incorporate a low-level communication systemfor communication between modules. The communication system may enablesynchronization of nearby luminaries to provide improved light qualitywhile conserving energy. Such operation may also be synchronized withdaylight harvesting. Different communication techniques may be used forthis, but various embodiments may use wired, wireless or opticalcommunication, where one or more light-emitting semiconductor dies 300are modulated at a high frequency to provide the communication signal.

In some embodiments of the invention, the above-described controlcircuits preferably include modulation/demodulation circuitry, and mayeven include circuitry such as a microprocessor, microcontroller, or thelike to process the transmitted and/or received communications. Thesignals may, for example, represent commands that adjust the operationof a master lighting system incorporating the modules 1600. Suitablenetwork and communication circuitry are well characterized in the artand a networked system of intercommunicating such lighting systems canbe straightforwardly configured without undue experimentation.

In various embodiments, each module 1600 may sense the state of itsnearest-neighbor modules 1600 (or other light-emitting fixtures) andtake some action based on what is sensed. For example, as depicted inFIG. 16B, a module A may sense a person in its local area. Thesurrounding modules B, C, D and E may not sense a person in their localareas, but do sense that A is emitting light. The control system may beprogrammed such that, for this situation, the desired light level in theareas illuminated by modules B, C, D, and E is 75% (compared to thelight level emitted by module A). The next-nearest-neighbors (not shown)may also not sense occupancy in their local areas, but sense that theirneighbors are emitting at 75%, and may thus emit at a value of, e.g.,50% of the nominal level. Extension of this scheme to multiple levels ofneighboring modules results in an autonomous system that detectsoccupancy and self-adjusts across modules to provide light around anoccupant but to turn off unneeded lights to save energy in a mannercomfortable for the occupant. Again, programmable control circuitry andsuitable sensors are conventional in the art, and may be programmed toachieve desired sensor-responsive illumination conditions (e.g., lightdrop-off patterns based on sensed occupancy) without undueexperimentation.

In the embodiment shown in FIG. 17, an electronic module 1700 (similarto module 1500 and/or 1600) operates as a backlighting unit (BLU)assembly for, e.g., a liquid crystal display (LCD) assembly. Thelighting module 1700 includes an array of light-emitting semiconductordies 300 (e.g., LEDs and/or lasers) adhered to a substrate 400 thatirradiate areas of phosphor 920 on a substrate 1240 (which is preferablyoptically transparent). (Flexure and/or deformation of substrate 400,any non-planarity of the semiconductor dies 300, traces 410 and theadhesive 510 have been omitted from various figures for clarity.) Thecombined light 1710 (including or consisting essentially of unconvertedlight emitted by semiconductor die 300 and/or light converted to adifferent wavelength by phosphor 920) is directed through one or moreoptical elements 1720 (e.g., Fresnel lenses) that may be embossed ormolded on a substrate 1730 (which is preferably optically transparent).The light 1710 then preferably illuminates an optical diffuser 1740. Thediffused light is then preferably directed through crossed brightnessenhancement films 1750, 1760 (e.g., Vikuiti BEF manufactured by 3MCorporation), which partially collimate and further diffuse the lightthat illuminates an LCD assembly 1770. In another embodiment,semiconductor die and phosphor 920 are integrated as shown in FIGS.8-11.

FIG. 18A depicts an electronic module 1800 that also operates as a BLUassembly for, e.g., an LCD assembly. The lighting module 1800 includesan array of light-emitting semiconductor dies 300 (e.g., LEDs and/orlasers) adhered to a substrate 400, light 1810 from which irradiates asubstrate 1820 that uniformly includes or consists essentially of aphosphor material (such as phosphor 920). Similarly to module 1700, thecombined and/or converted light is then directed through crossedbrightness enhancement films 1750, 1760, which partially collimate andfurther diffuse the light that illuminates LCD assembly 1770.

FIG. 18B depicts an electronic module 1801 that also operates as a BLUassembly for, e.g., an LCD assembly. The lighting module 1801 includesan array of light-emitting semiconductor dies 300 (e.g., LEDs and/orlasers) adhered to a substrate 400, a second substrate 800 with wells810 (see FIG. 8A) positioned over substrate 400 such that semiconductordie 300 are within wells 810, which are completely or partially filledwith phosphor 920. Light 1811 comprising light from semiconductor die300 and phosphor 920 is directed through crossed brightness enhancementfilms 1750, 1760, which partially collimate and further diffuse thelight that illuminates LCD assembly 1770.

FIGS. 19 and 20 depict electronic modules similar to modules 1700, 1800and 1801 that function as planar light sources for general illumination.As shown in FIG. 19, an electronic module 1900 includes an array oflight-emitting semiconductor dies 300 (e.g., LEDs and/or lasers) adheredto a substrate 400 that irradiate areas of phosphor 920 on a substrate1240 (which is preferably optically transparent). The combined light(including or consisting essentially of unconverted light emitted bysemiconductor die 300 and/or light converted to a different wavelengthby phosphor 920) is directed through one or more optical elements 1720(e.g., Fresnel lenses) that may be embossed or molded on a substrate1730 (which is preferably optically transparent). In another embodiment,semiconductor die and phosphor 920 are integrated as shown in FIGS.8-11.

Similarly, FIG. 20 depicts an electronic module 2000 that also includesan array of light-emitting semiconductor dies 300 (e.g., LEDs and/orlasers) adhered to a substrate 400. In module 2000, one or more of thesemiconductor dies 300 are “encapsulated” in phosphor 820 in the mannerdescribed above with reference to FIGS. 8A-8D. The combined light(including or consisting essentially of unconverted light emitted bysemiconductor die 300 and/or light converted to a different wavelengthby phosphor 820) may be directed through any of a variety of optics,e.g., the asymmetric Fresnel lenses 2010 and/or holographic diffuser2020 depicted in FIG. 20. The optics may be portions of, formed on,and/or bonded to a transparent substrate 2030. The lenses 2010 may bepositioned at a desired distance away from the semiconductor dies 300such that the image of each semiconductor die 300 substantiallyuniformly fills the exit pupil of its associated lens 2010 when viewedon-axis. All or a portion of light emitting die 300 may be associatedwith optical elements, for example, lenses. In one embodiment an arrayof light emitting die 300 is associated on a one-to-one basis with anarray of optical elements.

Referring to FIGS. 21, 22A, and 22B, in various embodiments, multiplesubstrates 400, each having one or more light-emitting semiconductordies 300 adhered thereto, are assembled together to form a module 2100that is a drop-in replacement for commercial lighting products. Eachsubstrate 400 and its associated semiconductor dies 300 may be assembledindependently of the other modules. The substrates 400 may be sorted (or“binned”) such that they possess similar or complementarycharacteristics, such as correlated color temperature, light output, andelectrical properties such as forward voltage.

As shown in FIGS. 22A and 22B, each substrate 400 features one or morelight-emitting semiconductor dies 300 (e.g., LEDs and/or lasers), andmay also be bonded to a substrate 800 containing regions of phosphor 820in the manner depicted in FIG. 8A-8D, 9A-9D, 10, or 11. The electricaltraces 410 may terminate in connection pads 2200 to facilitateelectrical connection of the semiconductor dies 300 to driving circuitry2210. The electrical connections for each string of semiconductor dies300 are preferably on one side of the substrate 400 (e.g., in the mannerdepicted in FIG. 7B) in order to separate the light-emission area ofmodule 2100 from drive circuitry 2210 and/or other electroniccomponents.

As shown in FIGS. 21-23, several substrates 400 may be assembledtogether to form a larger light-emitting module 2100. The substrates 400may be assembled together on a larger substrate 2220, which may haveoptical elements (e.g., discrete optics, diffusers, micro optics, and/orother optical elements) contained within and/or bonded or formedthereon. Substrate 2220 is also preferably transparent and may beyielding or substantially rigid. Preferably, the optical elementsinclude or consist essentially of lenses 2230 (such as Fresnel lenses)that are molded into a rigid substrate 2240 (and/or substrate 2220). Areflector 2250 may optionally be disposed on the top of at least aportion of module 2100 to reflect any light that is reflected from,e.g., substrate 2220.

The module 2100 may be mounted into a housing as shown in FIG. 23. Themodule 2100 may be attached to and/or placed within a rigid frame 2300(which may include or consist essentially of one or more substantiallyrigid materials, e.g., metal, plastic) to provide mechanical support. Apower supply 2310 for powering the semiconductor dies 300 and any othercircuitry (e.g., driving circuitry 2210, control circuitry, interfaces,etc.) may be disposed on a top surface 2320 of frame 2300. Thus mounted,the module 2100 may serve as a retrofit kit for existing luminaires in abuilding, a replacement luminaire for existing luminaires, or a newluminaire product for new construction. The thin form factor, optionallyless than approximately one inch in thickness, enables module 2100 to beused in many different situations. Packaged modules 2100 may have formfactors that match existing commercial installations, e.g., one foot byfour feet, two feet square (i.e., two feet by two feet), and/or two feetby 4 feet, or may have other shapes and form factors to meet variousdesign or lighting requirements.

FIGS. 24A and 24B depict the back and front sides, respectively, of amodule 2400 that may be utilized as a retrofit for, e.g., atwo-foot-square luminaire. FIG. 25 depicts a magnified cross-section ofthe module 2400 with many of the components (e.g., phosphors, optics,and drive circuitry) omitted for clarity. Individual substrates 400 maybe mounted (e.g., via an adhesive or mechanism such as a clamp) to asingle larger substrate 2410, which may include or consist essentiallyof, e.g., glass and/or plastic. The substrate 2410 may then be attachedto a large mechanical support sheet 2420 (that may include or consistessentially of a rigid material such as metal). An optional diffusereflector 2430 may be disposed between the substrate 2410 and mechanicalsupport sheet 2420. As shown, the above-described components are held ina c-channel-type extrusion 2440 by means of, e.g., screws 2450. Thescrews 2450 may also affix the small c-channel 2440 to larger c-channelextrusions 2460 running approximately perpendicularly across the back ofthe mechanical support sheet 2420. In this manner, the entire assemblymay be made mechanically rigid to prevent appreciable sag of thesubstrate 2410. The large c-channel extrusion 2460 also advantageouslyprovides a mechanical mounting point for power supplies and/or drivers2470 used to deliver the required voltage to the semiconductor dies 300and drive circuit boards 2480 at the perimeter of the array.

FIG. 26 shows a partially exploded view of a completed module 2400 thatincorporates a diffuser sheet 2600 and a steel frame 2610 that holds thelens for a typical two-foot-square fluorescent troffer luminaire.Standoffs 2620 may be used to set the distance between the diffusersheet 2600 and substrate 2410. As shown, the completed module anddiffuser sheet may be easily inserted into the frame 2610 and, onceassembled as shown in FIG. 27, may provide a simple and thin drop-insolution as a retrofit kit for fluorescent luminaires. FIGS. 28A and 28Bshow bottom views of the completed module 2400 in the steel frame withand without the diffuser sheet 2600, respectively.

EXAMPLES Example 1

Conductive traces 1 mm wide were formed on glass and polyethyleneterephthalate (PET) substrates, where the PET substrates had a thicknessof about 5 mils. The conductive traces included a bottom layer of Cr anda top layer of Au evaporated sequentially onto the substrate. The Crthickness was about 30 nm and the Au thickness was about 300 nm. Theconductive traces had gaps with a width of about 90 μm in positionswhere LEDs were to be attached. The LEDs were about 13 mils wide andabout 24 mils long and had two contacts on the same side of the die.Kyocera 0604C ACA was dispensed over the gap such that a portion of theend of each conductive trace adjacent to the gap, as well as the gapregion, was covered with ACA. The LED die was then placed, contact sidedown onto the ACA such that at least a portion of the n-contact was overat least a portion of the trace on one side of the gap and at least aportion of the p-contact was over at least a portion of the trace on theother side of the gap. The PET sheet with LEDs was then placed in a heatpress on a compliant pad with the LEDs facing up. A piece of glass wasplaced over the LEDs, and the heat-plate portion of the press wasapplied. The plate was set to 125° C. Pressure was applied and the PETsheet was left in the press for 10 min, then removed from the press andallowed to cool before removing the glass on the surface. Following theheat press operation, the sheet was dimpled where the LEDs were,indicating a deformation of the PET sheet during the process. The LEDdie attached to the PET substrates had 100% yield with respect toconduction, with no shorts or opens. LED die attached to the glass slidevia an equivalent process exhibited a large percentage (˜50% or more) ofintermittent contact failures.

Example 2

Conductive traces 1 mm wide were formed on PET substrates havingthicknesses of about 5 mils. Conductive traces were formed on thesubstrates by screen printing of silver ink. The height of the silverscreen-printed traces was about 4 μm. The conductive traces had gapswith widths of approximately 90 μm to 150 μm in positions where LEDswere to be attached. The LEDs were about 13 mils wide and about 24 milslong and had both contacts on the same side of the die. Kyocera 0604CACA was dispensed over the gap, such that a portion of the end of eachconductive trace adjacent to the gap, as well as the gap region, wascovered with ACA. The LED die was then placed, contact side down, ontothe ACA such that at least a portion of the n-contact was over at leasta portion of the trace on one side of the gap and at least a portion ofthe p-contact was over at least a portion of the trace on the other sideof the gap. The PET sheet with LEDs was then placed in a heat press on acompliant pad with the LEDs facing up. A piece of glass was placed overthe LEDs, and the heat plate portion of the press was applied. The heatplate was set to 125° C. Pressure was applied, and the PET sheet wasleft in the press for 10 min and then removed from the press and allowedto cool before removing the glass from the surface. As mentioned inExample 1, following the heat press operation, the sheet was dimpledwhere the LEDs were attached, indicating a deformation of the PET sheetduring the process. The LED die attached to the PET substrates had over99.8% yield with respect to conduction, with only shorts for the 0.2%failed LEDs for placement of over 7000 die.

Example 3

A device featured a LED emitting blue light adhered to a yieldingsubstrate as described above, and a phosphor mixture was disposed in awell surrounding the LED such that the light emitted from the device wassubstantially white with a specific nominal correlated color temperature(CCT) and a Color Rendering Index (CRI) of at least 75. The phosphormixture included 6% to 12% by weight yellow-emitting Al₅O₁₂Y₃:Ce²⁺phosphor (NYAG4563-S), 10% to 50% by weight (relative to the firstphosphor) amber-emitting (SrBaMg)₂SiO₄:Eu²⁺ phosphor (06040), 3% to 30%by weight (relative to the first phosphor) red-emitting CaAlSiN₃:Eu²⁺phosphor (R6535), and 1% to 5% by weight (relative to the firstphosphor) green-emitting (SrBaMg)₂SiO₄:Eu²⁺ phosphor (Y3957), all ofwhich are available from Intematix Corporation of Fremont, Calif.

The phosphor mixture was combined in the ratio of 1% to 5% by weight(relative to the first phosphor) with fumed silica (CAB-O-SIL CT-1221)available from Cabot Corporation of Billerica, Mass. in the ratio of 1%to 2% by weight with optically transparent silicone elastomer (Sylgard184) available from Dow Corning Corporation. The fumed silica (in otherembodiments fumed alumina is utilized in addition to or instead of fumedsilica) alleviates phosphor particle agglomeration and enhances theefficiency of light extraction from the phosphors. The phosphor mixturewas degassed and then injected into the wells. The mixture was injectedutilizing a 3 cc syringe with a tip size of 27 to 32 gauge, and thephosphor mixture is ejected by means of compressed air at 40 psi or amechanically-activated plunger. A thickness of 250 to 500 μm wasobtained by limiting the stroke length of the plunger or the applicationof compressed air to a predetermined time (e.g., 2 to 7 seconds).

Two different formulations of the phosphor mixture produced differentCCT values. The first mixture provided a CCT of 3500 K, and included 10%NYAG4653-S, 25% R6535, 3% fumed silica, and polydimethylsiloxane (PDMS)material having a refractive index of 1.43, and had a thickness ofapproximately 250 μm. The second mixture provided a CCT of 5000 K, andincluded 8.5% NYAG4653-S, 5% R6535, 3% fumed silica, and PDMS materialhaving a refractive index of 1.43, and had a thickness of approximately250 μm. In another embodiment the phosphor binder was Dow OE-6550 with arefractive index of approximately 1.53.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

1. An electronic device comprising: an unpackaged inorganiclight-emitting diode (LED) die having first and second distinctnon-coplanar contacts on a first surface thereof; and a flexiblesubstrate having first and second conductive traces on a first surfacethereof, the first and second conductive traces being separated on thesubstrate by a gap therebetween, wherein the first and second contactsare adhered to and in electrical contact with, respectively, the firstand second conductive traces with a pressure-activated adhesive materialnotwithstanding the non-coplanarity of the first and second contacts,and without electrically bridging the traces or the contacts.
 2. Thedevice of claim 1, wherein the substrate is flexible and deformable. 3.The device of claim 1, wherein the inorganic LED die comprises asemiconductor material comprising at least one of GaN, AlN, InN, or analloy or mixture thereof.
 4. The device of claim 1, wherein the adhesivematerial comprises an anisotropic conductive adhesive (ACA) electricallyconnecting the first contact only to the first trace and the secondcontact only to the second trace.
 5. The device of claim 4, wherein aportion of the ACA is disposed in the gap and substantially isolates thefirst contact from the second contact.
 6. The device of claim 1, whereina thickness of the first trace and a thickness of the second trace aresubstantially uniform and substantially equal to each other.
 7. Thedevice of claim 1, further comprising a reflective material over atleast a portion of the first surface of the inorganic LED die.
 8. Thedevice of claim 1, wherein an offset between the first and secondcontacts along a dimension substantially perpendicular to the firstsurface of the inorganic LED die, is at least 0.25 μm.
 9. The device ofclaim 1, wherein the substrate comprises a localized deformation betweenthe first and second traces, whereby a distance between the firstcontact and the substrate is substantially equal to a distance betweenthe second contact and the substrate.
 10. The device of claim 1, whereinthe inorganic LED die extends across the gap between the first andsecond traces, and further comprising a second inorganic LED die,proximate the inorganic LED die, extending across the gap between thefirst and second traces.
 11. The device of claim 1, wherein the firstand second conductive traces comprise a conductive ink.
 12. The deviceof claim 1, wherein the first and second conductive traces comprise atleast one of silver, gold, aluminum, chromium, copper, or carbon. 13.The device of claim 1, wherein the substrate comprises at least one ofpolyethylene naphthalate, polyethylene terephthalate, polycarbonate,polyethersulfone, polyester, polyimide, polyethylene, or paper.
 14. Thedevice of claim 1, wherein the gap between the first and second tracesranges between approximately 25 μm and approximately 1000 μm.
 15. Thedevice of claim 1, wherein there is no heat sink in thermalcommunication with the inorganic LED die.
 16. The device of claim 1,further comprising, disposed over at least a portion of the inorganicLED die, a phosphor material for converting at least a portion of lightemitted by the inorganic LED die to light of a different wavelength. 17.The device of claim 16, further comprising an optically transparentmaterial disposed between the inorganic LED die and the phosphormaterial.
 18. The device of claim 16, further comprising, disposed overthe phosphor material, a reflective surface for reflecting convertedlight toward the flexible substrate.
 19. The device of claim 16, furthercomprising a second substrate disposed over the flexible substrate andthe first and second conductive traces, the second substrate comprisingan opening defined thereby, the inorganic LED die and the phosphormaterial being disposed in the opening.
 20. The device of claim 19,further comprising a transparent film disposed over the opening in thesecond substrate.
 21. The device of claim 19, wherein the secondsubstrate is flexible.
 22. The device of claim 1, wherein a side lengthor diameter of each of the first and second contacts is less than 70 μm.23. The device of claim 1, wherein a total area of the first surface isat least 15 times an area of the first contact.
 24. The device of claim1, wherein the adhesive material comprises a substantially isotropicadhesive electrically connecting the first contact only to the firsttrace and the second contact only to the second trace, and furthercomprising a non-conductive adhesive material disposed in the gap. 25.The device of claim 1, wherein a reflectivity of the substrate for awavelength emitted by the semiconductor die is greater than 80%.
 26. Thedevice of claim 1, wherein a transmittance of the substrate for awavelength emitted by the semiconductor die is greater than 80%.